In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) in addition to the early voice-oriented services.
In the communication systems, a receiver and a transmitter need to be synchronized with each other for data reception and demodulation. In particular, in order for the communication systems to successfully receive data in a continuously changing channel environment, synchronization is adjusted through closely-related signaling between a base station (BS) and a user equipment (UE).
A communication channel between the BS and the UE includes a downlink channel directed from the BS to the UE and an uplink channel directed from the UE to the BS.
In downlink, a plurality of UEs is downlink synchronized according to a data frame transmitted from the BS in a point-to-multipoint environment. According to a method for allowing the UEs to attain synchronization, the BS can insert a preamble for synchronization into a portion of the transmitted frame. The UEs adjust synchronization for a downlink channel by using the preamble. The BS can use a separate synchronization channel.
In uplink, each UE has to transmit data to the BS through time and/or frequency domains assigned to each UE so that the BS can receive data without interference between the UEs. Therefore, for uplink synchronization, there is a need to regulate synchronization through signaling between the BS and the UE by considering a channel environment of each UE. A signal exchanged between the UE and the BE to adjust uplink synchronization is referred to as a ranging signal. A series of processes for controlling transmit (Tx) power by exchanging the ranging signal between the UE and the BS and for adjusting time/frequency synchronization is referred to as a ranging procedure. It can be said that the ranging procedure is a series of processes for attaining uplink synchronization.
Initial ranging is a process for obtaining an exact timing offset between the UE and the BS and for regulating Tx power in an initial stage. When power is turned on, the UE attains downlink synchronization from a received downlink preamble signal. Subsequently, the UE performs the initial ranging to obtain an uplink timing offset and to regulate the Tx power. Unlike the initial ranging, periodic ranging is a process for periodically tracking the uplink timing offset and received signal strength after the initial ranging.
If the ranging procedure is delayed, uplink transmission of the UE is also delayed. In addition, since data is not actually transmitted in the ranging procedure, repetition of the ranging procedure may lead to waste of battery capacity of the UE irrespective of data transmission.
Accordingly, there is a need for a method for rapidly attaining uplink synchronization while minimizing battery consumption of the UE.